The present invention relates to viscous fluid couplings, and more particularly, to such couplings which are used to drive vehicle radiator cooling fans, wherein the engagement or disengagement of the coupling is controlled in response to an input signal, which typically represents the temperature of the engine coolant.
In a typical viscous fluid coupling (viscous fan drive) of the type to which the present invention applies, an input shaft drives an input coupling member (clutch) which is received within an output coupling assembly, and torque is transmitted from the input to the output, in the presence of viscous fluid, by means of viscous shear drag. The coupling normally includes some sort of valving which controls the amount of viscous fluid within a viscous shear chamber, thereby controlling the ratio of the output torque and speed to the input torque and speed. Typically, the valving comprises a valve member which is moveable to cover or uncover a fill port disposed between a reservoir and the viscous shear chamber (operating chamber).
As is well known to those skilled in the art, the transmission of torque from an input coupling member to an output coupling member by means of viscous shear drag results in the generation of a substantial amount of heat. At least a major portion of such heat must be dissipated, or else the temperature of the viscous fluid will continue to increase, as the fan drive operates, until the fluid eventually breaks down (longer polymer chains break up into shorter polymer chains) and eventually, the viscosity of the fluid begins to decrease, as does its ability to transmit torque.
As is also well known to those skilled in the art, the coefficient of heat transfer of a heat generating device, such as a viscous fluid coupling, is on the order of fifty times greater when the device is surrounded by water than when it is surrounded by air. In other words, it would be possible to dissipate more heat from a viscous coupling, and therefore, transmit more torque through the coupling, if it were surrounded by water, instead of being disposed in the air stream rearward of the vehicle radiator, as in the conventional arrangement.
U.S. Pat. No. 2,838,244, assigned to the assignee of the present invention and incorporated herein by reference, discloses a fan drive system including an input pulley, a viscous fan drive, and a radiator cooling fan. In the system of the '244 patent, the viscous fan drive is disposed partially within the water pump cavity of an internal combustion engine. Unlike the conventional viscous fan drive, in the device of the '244 patent, the outer coupling member is the input, and the coupling member disposed therein comprises the output of the fan drive. The entire inside of the outer coupling member contains viscous fluid at all times, and the torque transmitted is varied by moving the output coupling member axially, thereby varying the viscous shear area. Such movement of the output coupling member occurs in response to temperature variations, by means of a wax pill. The input coupling assembly includes impeller blades, such that the rearward portion of the input coupling assembly comprises the engine water pump impeller.
Although it should be apparent to one skilled in the art that the general arrangement of the cited patent will provide substantially improved heat dissipation from the viscous coupling, applicant is not aware of any commercially produced viscous fan drive being made in accordance with the teachings of the cited patent.
It should be understood that the viscous fluid coupling of the present invention has, as an essential feature, that it be water-cooled, i.e., that it be cooled by the flow of engine coolant, and the terms "water" and "coolant" will be used interchangeably hereinafter, it being understood that the particular composition of the fluid is not an essential feature of the invention. It is also not an essential feature of the invention that the viscous coupling comprise the engine water pump, although the invention is especially advantageous when the viscous coupling and the water pump are combined, and therefore, the present invention will be described in connection with such an embodiment.